(1) Field of the Invention
The present invention relates to a multilayer ceramic capacitor. More particularly, the present invention relates to a multilayer ceramic capacitor comprising internal electrodes composed of nickel and termination electrodes which are excellent in adhesion to the internal electrodes and also in adhesion to a dielectric body.
(2) Description of the Prior Art
Commercially available multilayer ceramic capacitors are fabricated by laminating a plurality of thin dielectric bodies having an internal electrode formed on the surface thereof, integrally sintering the laminate, and connecting the internal electrodes in parallel alternately to a pair of termination electrodes (electrodes for external connection) formed on the side faces of the sintered body.
When a noble metal such as palladium (Pd) or silver-palladium (Ag-Pd) is used for the internal electrode, the total cost of the capacitor is increased. Accordingly, a trial has recently been made to use nickel (Ni), which is a relatively cheap base metal, as the material of the internal electrode. Accordingly, investigations have been made to find a metal for the termination electrodes, having a good compatibility with nickel (Ni) used for the internal electrode.
In multilayer ceramic capacitors, silver (Ag) and silver-palladium (Ag-Pd) is ordinarily used as the termination electrodes metal. However, when a noble metal such as silver (Ag) or silver-palladium (Ag-Pd) is used for the termination electrodes, the total cost of the capacitor is increased, and since silver (Ag) or silver-palladium (Ag-Pd) is poor in compatibility with nickel (Ni) as the internal electrode, the connection between the internal and termination electrodes is insufficient and there are brought about fatal defects of electric characteristics, such as reduction of the electric capacity of the capacitor and increase of the dielectric loss (tan .delta.). As means for obviating this disadvantage, there may be considered a method in which a metal having a good compatibility with nickel (Ni) of the internal electrode and providing a good bonding between electrodes, that is, the same metal (Ni) as the metal of the internal electrode or a metal capable of being easily alloyed with the internal electrode, such as copper, iron or cobalt, is used as the termination electrodes. However, in this case, there arises a problem concerning the glass component to be mixed with this metal.
For example, it is known that lead borosilicate or bismuth borosilicate is mixed as the glass component with silver (Ag) or silver-palladium (Ag-Pd) and the mixture is coated on a dielectric body and sintered in air. However, if lead borosilicate glass or bismuth borosilicate glass is mixed with nickel (Ni) or a metal capable of being easily alloyed with nickel (Ni) and the mixture is sintered, nickel (Ni) or the metal capable of being easily alloyed with nickel (Ni) is oxidized, though this oxidation is not caused in case of silver (Ag) or silver-palladium (Ag-Pd), and this oxidation results in reduction of the capacity of the capacitor and increase of the dielectric loss. As means for obviating this disadvantage, there may be considered a method in which a mixture of nickel (Ni) or the metal capable of being easily alloyed with nickel (Ni) and lead borosilicate glass or bismuth borosilicate glass is sintered in a non-oxidizing atmosphere. As indicated by curves l.sub.1 and l.sub.2 in FIG. 2, lead oxide (PbO) and bismuth oxide (Bi.sub.2 O.sub.3) are poorer in the thermal stability than Al.sub.2 O.sub.3 and SiO.sub.2 indicated by curves l.sub.3 and l.sub.4 in FIG. 2. Accordingly, for example, at a temperature of 900.degree. C., if the oxygen partial pressure (PO.sub.2) in the sintering atmosphere is reduced below about 10.sup.-10 atmosphere, lead oxide and bismuth oxide releas oxygen, that is, they are reduced. Accordingly, in the non-oxidizing atmosphere, lead oxide (PbO) or bismuth oxide (Bi.sub.2 O.sub.3) in the glass component is reduced and the glass becomes brittle, with the result that the strength of the termination electrodes per se is reduced, the strength of fixing to the dielectric body is reduced, the adhesion between the electrodes becomes insufficient and reduction of the capacity and increase of dielectric loss are caused.